Mean curvature flow without singularities

We study graphical mean curvature flow of complete solutions defined on subsets of Euclidean space. We obtain smooth long time existence. The projections of the evolving graphs also solve mean curvature flow. Hence this approach allows to smoothly flow through singularities by studying graphical mean curvature flow with one additional dimension.Comment: 19 pages, 4 figure

The politics of the street: Street art, public writing and the history of political contest in Chile

This commentary focuses on the politics of public space in democracy and dictatorship. It delves into what Peter Winn calls the revolution ‘from below’ from the perspective of urban conflict, suggesting a political history that attends to urban and visual culture as a crucial arena of political practice. It suggests that the often-conflictive battle over public spaces was, and continues to be, a mechanism by which an unprecedented range of citizens entered into an ongoing debate over the boundaries of citizenship, practice, politics and that this practice was adapted, transformed and reimagined over the last five decades. The struggle over streets and walls continues to be central to Chilean political history, and urban space remains a field of ongoing contest and debate: the estallido of social unrest in contemporary Chile connected a new generation of activists to this longer history of creative politics of protest and protest art and gave them the opportunity to articulate new forms of intersectional political thought in public space, even in the face of state-sponsored violence. Studying these forms of unrest reveals that theirs is an incisive, intersectional critique of the limits of the ‘transition to democracy’, of neoliberal democracies and of the legacies of dictatorship

On the Presence of Thermal SZ Induced Signal in the First Year WMAP Temperature Maps

Using available optical and X-ray catalogues of clusters and superclusters of galaxies, we build templates of tSZ emission as they should be detected by the WMAP experiment. We compute the cross-correlation of our templates with WMAP temperature maps, and interpret our results separately for clusters and for superclusters of galaxies. For clusters of galaxies, we claim 2-5 $\sigma$ detections in our templates built from BCS Ebeling et al. (1998), NORAS (Boehringer et al. 2000) and de Grandi et al. (1999) catalogues. In these templates, the typical cluster temperature decrements in WMAP maps are around 15-35 $\mu$K in the RJ range (no beam deconvolution applied). Several tests probing the possible influence of foregrounds in our analyses demonstrate that our results are robust against galactic contamination. On supercluster scales, we detect a diffuse component in the V & W WMAP bands which cannot be generated by superclusters in our catalogues (Einasto et al. 1994, 1997), and which is not present in the clean map of Tegmark, de Oliveira-Costa & Hamilton (2003). Using this clean map, our analyses yield, for Einasto's supercluster catalogues, the following upper limit for the comptonization parameter associated to supercluster scales: y_{SC} < 2.18 \time s 10^{-8} at the 95% confidence limit.Comment: MNRAS accepted. New section and minor changes include

Broken discrete and continuous symmetries in two dimensional spiral antiferromagnets

We study the occurrence of symmetry breakings, at zero and finite temperatures, in the J_1-J_3 antiferromagnetic Heisenberg model on the square lattice using Schwinger boson mean field theory. For spin-1/2 the ground state breaks always the SU(2) symmetry with a continuous quasi-critical transition at J_3/J_1=0.38, from N\'eel to spiral long range order, although local spin fluctuations considerations suggest an intermediate disordered regime around 0.35 < J_3/J_1 < 0.5, in qualitative agreement with recent numerical results. At low temperatures we find a Z_2 broken symmetry region with short range spiral order characterized by an Ising-like nematic order parameter that compares qualitatively well with classical Monte Carlo results. At intermediate temperatures the phase diagram shows regions with collinear short range orders: for J_3/J_11 a novel phase consisting of four decoupled third neighbour sublattices with N\'eel (\pi,\pi) correlations in each one. We conclude that the effect of quantum and thermal fluctuations is to favour collinear correlations even in the strongly frustrated regime.Comment: 17 pages, accepted for publication in Journal of Physics: condensed Matte

Classical Antiferromagnetism in Kinetically Frustrated Electronic Models

We study the infinite U Hubbard model with one hole doped away half-filling, in triangular and square lattices with frustrated hoppings that invalidate Nagaoka's theorem, by means of the density matrix renormalization group. We find that these kinetically frustrated models have antiferromagnetic ground states with classical local magnetization in the thermodynamic limit. We identify the mechanism of this kinetic antiferromagnetism with the release of the kinetic energy frustration as the hole moves in the established antiferromagnetic background. This release can occurs in two different ways: by a non-trivial spin-Berry phase acquired by the hole or by the effective vanishing of the hopping amplitude along the frustrating loops.Comment: 12 pages and 4 figures, with Supplementary Material. To be published in Phys. Rev. Let

A test of the bosonic spinon theory for the triangular antiferromagnet spectrum

We compute the dynamical structure factor of the spin-1/2 triangular Heisenberg model using the mean field Schwinger boson theory. We find that a reconstructed dispersion, resulting from a non trivial redistribution of the spectral weight, agrees quite well with the spin excitation spectrum recently found with series expansions. In particular, we recover the strong renormalization with respect to linear spin wave theory along with the appearance of roton-like minima. Furthermore, near the roton-like minima the contribution of the two spinon continuum to the static structure factor is about 40 % of the total weight. By computing the density-density dynamical structure factor, we identify an unphysical weak signal of the spin excitation spectrum with the relaxation of the local constraint of the Schwinger bosons at the mean field level. Based on the accurate description obtained for the static and dynamic ground state properties, we argue that the bosonic spinon theory should be considered seriously as a valid alternative to interpret the physics of the triangular Heisenberg model.Comment: 6 pages, 5 figures, extended version including: a table with ground state energy and magnetization; and the density-density dynamical structure factor. Accepted for publication in Europhysics Letter

An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxies

We present an XMM-Newton observation of the bright, narrow-line, ultrasoft Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parameterised using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionised accretion disc, inverse-Compton scattering of soft disc photons by thermal electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal distribution. We emphasise the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc.Comment: 9 pages, accepted for publication in MNRA